The Meaning of Life, the Universe, and Everything.
when you go mining in minecraft, you've filled your inventory with things you might think are "useful" up above, on the surface, but when you find something else useful maybe in a dungeon, you can't fit it in there because you already jammed your inventory full of useful things. so you have to go through your inventory and decide which things you need to sacrifice to fit that other item in there. But backpacks make your inventory 1 and 1/3 rows bigger so you can fit more stuff in when you go mining or exploring
I agree with Bloody Phoenix, there are several methods for reducing and managing one's inventory already built into the game. We don't really need anything more
Also, no matter how large they make the inventory, some people will also always collect to the limit... any then want even more space in the inventory. It's a vicious circle really.
However, I do like the look of a backpack on Steve (they're cute) ... and actually feel that it is the only (semi)logical way in which he carries his already significant inventory already (I mean, he's certainly not carrying everything in his one hand, is he?)... So, I think they should just add them to every skin as standard startup equipment (without the need to craft them or necessarily increasing the size of the inventory). Don't really think that'll happen, though.
I PC gamers have the options of mods if they genuinely need to increase their inventory sizes, however they also have access to mods that introduce more items that may be deemed necessary, thus reducing their free inventory space more than a console gamer.